In Long Term Evolution (LTE) uplink, a Single Carrier-Frequency Division Multiple Access (SC-FDMA) waveform is used and the time-frequency resources for the Physical Uplink Shared Channel (PUSCH) can be allocated by a single or dual cluster of frequency resources, where each cluster (i.e., set of frequency resources) is localized in frequency within a slot and consists of a number of frequency-consecutive Physical Resource Blocks (PRBs). A single Discrete Fourier Transform (DFT)-precoder is applied for the one or two clusters. Hence, a single cluster achieves the lowest Cubic Metric (CM) and Peak-to-Average-Power Ratio (PAPR) performance, while a dual cluster allocation provides slightly more freedom for the scheduler, albeit at potentially larger CM/PAPR. Dynamic PUSCH resource allocation is conveyed by a resource allocation field in the associated uplink grant sent in the downlink control channel, where PUSCH resource allocation type 0 is used for single cluster PUSCH, and PUSCH resource allocation type 1 is for dual cluster PUSCH.
LTE Rel-13 can also be deployed for downlink transmissions in unlicensed spectrum, i.e., utilizing Licensed Assisted Access (LAA) where an unlicensed carrier is operated as a Secondary Cell (SCell) in conjunction with a Primary Cell (PCell) located in licensed spectrum. It is desirable to extend the functionality of LAA by including uplink (UL) transmissions. In particular, LTE relies on E-UTRAN NodeB or evolved NodeB (eNodeBs) to perform uplink scheduling, which allows multiple User Equipments (UEs) in a cell to transmit PUSCH on orthogonal resources within a subframe. That is, LTE is not constrained to wideband scheduling for one user at a time, as is the case for many WiFi systems, and could leverage the frequency selectivity of the channels for the UEs into scheduling gains. The scheduler could also schedule UEs on the same time-frequency resource within a cell and utilize spatial suppression to separate the UEs at the receiver, i.e., Multi-User MIMO (MU-MIMO).
For LAA, a first regulatory requirement is that the occupied channel bandwidth shall be between 80% and 100% of the declared nominal channel bandwidth. The occupied channel bandwidth is the bandwidth containing 99% of the power of the signal. This requirement does not mandate that only a single UE can occupy 80-100% of the carrier bandwidth. For example, it would be possible to multiplex PUSCH from several UEs in an UL subframe over the whole carrier bandwidth using interleaved Frequency Division Multiplexing (FDM) allocation, while fulfilling the occupied channel bandwidth requirement. In addition, a second regulatory requirement is the transmission power in a narrow band. For example, in the frequency band 5250-5350 MHz, the power spectral density for transmissions shall be limited to a maximum mean Equivalent Isotropically Radiated Power (EIRP) density of 10 mW/MHz in any 1 MHz band. This implies that, in order not to limit the transmit power, it is beneficial to allocate the resources in as many ‘1 MHz’ bands as possible.
In principle, using a large single cluster in PUSCH resource allocation could guarantee that the channel bandwidth occupancy requirement is met for a UE as well as the maximum mean EIRP is not exceeded. However, this would result in an inefficient system operation since it may imply either that only one UE could be scheduled at a time or that the code rate will be very low since a large amount of contiguous resources has to be allocated. In order to efficiently support UE multiplexing of PUSCH, extending the current single and dual cluster allocation to allow multi-cluster (>2) allocation (e.g., PRBs/subcarriers spaced uniformly in frequency) has been identified as a candidate waveform that satisfies regulatory requirements. Furthermore, an efficient resource allocation scheme should allow UEs to be allocated with different amount of resources, e.g., different number of PRBs. At the same time, it is important that the resource allocation information can be signalled to the UE with few bits, in order to reduce the overhead on the downlink control channel. It is therefore an open issue to define multi-cluster allocation for PUSCH transmission for LAA and the corresponding encoding of the allocation information.
In LTE, the starting resource index, i.e., PRB index, and the number of allocated resources, i.e., number of PRBs, are represented by a single integer value which is signalled to the UE. PUSCH resource allocation type 0, i.e., single cluster PUSCH, comprises encoding through the resource index, a starting PRB index and the number of allocated PRBs. PUSCH resource allocation type 1, i.e., dual cluster PUSCH, comprises encoding through the resource index, four RBG indices, where the first two RBG indices are used for the starting RBG index and the ending RBG index of one cluster, and the last two RBG indices are used for the other cluster in the same way. These methods cannot support a multi cluster PUSCH, as the current PUSCH resource allocation only supports up to two clusters. Furthermore, the number of allocated resource blocks, N_PRB, is constrained by N_PRB=2α2·3α3·5α5≤NULRB, where α2,α3,α5 are non-negative integers and NULRB is the number of available PRBs.
In one conventional solution, the multi cluster PUSCH is indicated using 10 clusters with 1 PRB per cluster and an inter-cluster spacing of 1.8 MHz (10 PRBs), i.e., every 10th PRB is allocated. Thus there are 10 different PRB allocations, each comprising 10 PRBs. The UE may be allocated from 1 to 10 of these PRB allocations, yielding from 10 to 100 PRBs in total. The exact assignment of which resource allocation to be used is left to eNB, e.g., by signalling via a 10-bit bitmap in the UL grant. The signalling overhead is thus 10 bits to indicate the multi-cluster PUSCH.